Reduction of tube seal heating in high-frequency apparatus



March 28, 1950 Filed Nov. 28, 1947 OUTPUT F. GO 2,502,144

W. ETTER REDUCTION OF TUBE SEAL HEATING IN HIGH-FREQUENCY APPARATUS 2 Sheets-Sheet 1 Inventor. WiHiavh F Goet ter;

YMDW H is Attofin ey.

March 28, 1950 w. F. GOETTER REDUCTION OF TUBE SEAL HEATING IN HIGH-FREQUENCY APPARATUS 2 Sheets-Sheet 2 Filed NOV. 28, 1947 Inventor: William F Goettev;

His Attorhey.

Patented Mar. 28, 1950 UNITED STATES PATENT OFFICE REDUCTION OF TUBE SEAL HEATING IN HIGH -FREQUEN CY APPARATUS William F. Goetter, Liverpool, N. Y., assignor to General Electric Company, a corporation of New York Application November 28, 1947, Serial No. 788,451

3 Claims.

of the tubes in a very high frequency apparatus of the transmission-line-tuned type.

In recent years there has been increasing emphasis upon the development of high power radio transmitting equipment for operation in the socalled very-high-frequency bands, particularly those assigned for television and frequency modulation broadcasting service. In the development of equipment for operation at these frequencies, whichmay for example extend from frequencies of the order of 40 megacycles up to frequencies of more than 200 megacycles, there are a number of factors affecting both vacuum tube design and transmitter design which limit the amount of useful radio frequency power which can be generated and transmitted. For example, in the push-pull, transmission-line-tuned type of circuit utilizing air-cooled tubes, the tubes must be of relatively large physical size, with at-- calized heating is the non-uniform distribution of capacity currents through the tube. Due to the high inter-electrode tube capacitances, these currents may be very large for even moderate voltages at these operating frequencies. The well-known skin effect limits the depth of penetration of the current into the metal electrodes of the tube so that most of this current must pass over the electrode surfaces adjacent the glass-to-metal seals. These currents therefore tend to be concentrated in certain areas, for reasons that will become more fully apparent 'as the description of 'my invention proceeds, causing localized heating effects which must not be allowed to exceed the maximum safe temperature to which the glass tube envelope may be subjected.

Itis a principal object of my invention to provide an improved structure for high frequency apparatus which reduces such undesirable local- 2 ized seal heating and thereby permits higher power output.

It is particularly an object of my invention to provide means for reducing seal heating due to non-uniform capacity currents of tubes in very high frequency apparatus.

A still more specific object of my invention is to provide an improved push-pull circuit structure of the transmission-line-tuned type, particularly suitable for a power stage in a very high frequency radio transmitter.

For additional objects and advantages, and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings, and also to the appended claims in which the features of the inven-e tion believed to be novel are particularly pointed out.

In the drawings:

Fig. 1 is a conventionalized perspective view, largely in diagrammatic form, of a high-frequency, push-pull amplifier circuit embodying my invention;

Fig. 2 is a side elevational view, partly in section, showing certain essential structural elements of the amplifier of Fig. 1;

Fig. 3 is a horizonal cross-sectional view along the line 3-3 of Fig. 2;

Fig. 4 is a, horizontal cross-sectional view through one of the tube structures of Fig. 2 along the line 4-4; and

Figs. 5 and 6 are flux plots which will be referred to for better understanding of the operation of the circuit of Fig. 1. In the several figures of the drawing, corresponding elements have been indicated by corresponding reference numerals.

The amplifier of Fig. 1 is represented for purposes of illustration as being of the high-power, grounded-grid type comprising a pair of triode discharge devices In and I l. circuit comprises a pair of vertically-positioned, parallel transmission line conductors l2 and I3, and the tuned cathode circuit comprises a similar pair of horizontally-positioned, parallel transmission line conductors l4 and IS.

The upper ends of the anode line conductors I 2 and I3 are connected directly to the anodes l6 and I! of the-devices I0 and II respectively. For purposes of tuning and impedance matching, the usual adjustable short-circuiting member l8 and trimming capacitor l9 are provided. Anode operating potentials are impressed on the shortcircuited end of the transmission line from a suitable source, indicated conventionally by 3+,

through a filter comprising choke 20 and by-pass The tuned anode 3 capacitor 2|. Radio frequency power is coupled to any suitable output circuit (not shown) by means of the coupling loop 22 which is also provided with the usual short circuiting member 23 for making tuning adjustments.

The control grids 24 and 25 of the amplifiers l and H are connected effectively to ground, or neutral, potential through a bypass capacitor 26, and suitable self-bias potentials are developed by the resistance-capacitance network 21 in con ventional manner.

The length of the open-wire transmission line comprised by conductors I4 and I is determined by the position of an adjustable short-circuiting member 28 in the same manner as for the anode line. The open end of the line conductors l4 and [5 are connected respectively to the. cathodes 29 and 3%, represented as of the filamentary type,

by means of the pairs of coupling capacitors Ma, No, and 32a, and 32b respectively. High frequency input signals to be amplified are coupled into the transmission line l4, [5 from any suit able source (not shown) by means of the coupling loop 33 which. like the output coupling loop 22, is provided with an adjustable shorting member 34 for tuning purposes. A balancing and impedance-matching capacitor 35 is also shown connected across the open end of transmission line l4, IS in accordance with conventional practice. The cathodes 29 and 30 of tubes Ill and H are supplied with suitable heating potentials by connections to the filament supply transformers 40 and 41 which are energized in conventional manner from any suitable sources (not shown).

It will be apparent to those skilled in the art conductors i2 and [3 will generally be of relatively large physical size and spaced relatively close together. One suitable structural arrangement of these elements of the system is illustrated in Figs. 2 and 3 in which elements corresponding to those in Fig. 1 have been designated by the same reference numerals. The transmis sion line conductors l2 and i3 are of the tubue lar type and mounted upon a common metal shelf 43 which is insulated from ground in any suitable manner (not shown) and connected to 3+. The triodes l0 and II are of the air-cooled type in which the anodes l6 and I! are located on axes of the transmission line conductors and surrounded by external electrode structures comprising cylinders Ilia and Ila. These cylinders are illustrated as being of the same diameters as the transmission line conductors and they are supported directly upon the upper ends thereof, thereby effectively constituting a continuation of the line conductors. They may readily be centered and connected electrically to the transmission line conductors by means of the removable clamping rings 44 and 45.

The grid terminals of the tubes I!) and II in Fig. 2 are represented at 24a and 250. while the pairs of filament terminals are represented at 29a. and 30a respectively. The anodes I t and I! are surrounded by plurality of radial heat dissipating fins, such as are illustrated at I'Ib in Fig. 4., which connect them electrically to the cylinders [6a and Ma. Although my invention is notlimited' in its application to tubes of this specific type, the tubes l0 and II may for ex ample be of the type known commercially as (EL-5518, to which reference may also be made for additional details of construction. Cooling fluid, such as air, may be circulated through the hollow transmission line cylinders and over the tube structures as indicated by the dashed arrows in Fig. 2. It will be appreciated that the tubes may alternatively be of the liquid-cooled type, in which case suitable connections will be provided for circulating water or other liquid coolant around the tube anodes.

Fig. 5 is a partial flux plot showing electrostatic lines of force 48 and electromagnetic lines of force 49 surrounding a pair of conductors spaced relatively close together as compared to their diameters. When voltages of opposite polarity with respect to the neutral plane 46 are impressed upon these conductors in free space, the flux distribution is symmetrical with respect to this plane and with respect to the plane 41 through the axes of the conductors, as is well known. Therefore the fiux plot is shown for only one quadrant. It will be noted that the distribution of the electrostatic lines of force 48, at their terminations upon the surfaces of the conductors, is non-uniform, the most intense field existing in the space between the two conductors. On a uniform transmission line, it is fundamental that the current flow on the surface of the conductor is distributed exactly as the distribution of electrostatic lines of force which terminate on the conductor. The arrangement as shown therefore indicates that there will be a high concentration of current on the adjacent surfaces of the conductors and a relatively low concentration on the outside surfaces of the conductors.

In the high power amplifier structure thus far described, this general type of flux and current distribution at the anode end of the combined transmission line and electrode structure causes a correspondingly non-uniform concentration of high frequency currents flowing radially inward to the tube anodes i5 and I1. Due to skin effects, as previously mentioned, and also due to dielectric heating effects within the glass en- 1 velopes themselves at these high frequencies, the

seal heating may be so severe at points of high current concentration adjacent the glass-tometal seals 36 and 31 (Fig. 2) as to cause tube failure.

In accordance with my invention, this localized seal heating is greatly reduced, and the maximum useful power output of the system greatly increased, by means of the pair of conducting plates or shields 51. 52 which partially surround the transmission line conductors and anode electrode structures, as shown in Figs. 1, 2 and 3. These plates are effectively maintained at ground, or'neutral, potential for high frequencies by connection at their upper ends to ground through capacitor 53 and by connection at their lower ends to the grounded ends of the transmission line conductors l2 and I3. As is best shown in Fig. 2, the bypass capacitor 53 may structurally comprise a grounded supporting plate 54 and a conducting plate. 55 spaced therefrom. by means The upper ends of the plates 5! and 52 are then bonded se of a sheet of dielectric material 55.

curely to the plate 55 in any suitable manner,

as by means of brackets 5'! and 53 respectively. The grid bypas capacitor 26 may also be of simi-- terial 60. All of the capacitor plates and dielec;

trio sheets are provided with a pair of circular atom-44 apertures 38 and 39 registering respectively with the tubes l and II, and the plate 59 is provided withaplurality of spring contact fingers 6| and 62 radially arranged around each aperture for providing good electrical contact with the grid terminals 24a and 25a.

The lower ends of the plates and 52 may conveniently be supported by means of conductive studs 66 and 61, as best shown in Fig. 3. In Figs. 2 and 3, the shorting member I8 is illustrated as comprising the pair of clamping bars |8a and |8b which are shaped to make good electrical contact with the conductors |2 and I3 and held in position by the wing bolt 68.

The effect of the plates 5| and 52 upon the flux distribution is illustrated by the flux plot of Fig. 6. The plates 5| and 52 have arbitrarily been illustrated in the form of partial cylinders each of which is concentric with one of the transmission line cylinders l2 and I3. Each plate is also illustrated as subtending an angle of approximately 120 degrees and as being spaced from the adjacent cylinder by a distance approximately equal to one half the radius of the cylinder. However, these configurations and spacings are not critical.

Fig. 6 shows that the plates 5| and 52 cause the flux concentration over the surfaces of the conductors l2 and I3 to be substantially the same over the areas adjacent the shield plates 5| and 52 as over the areas lying between the transmission line conductors. The flux concentration over the rest of the circumference is somewhat less than this, but never less than about One half the maximum concentration. Therefore, the capacity current distribution to the tube anodes is correspondingly much more uniform than under the conditions previously illustrated in connection with the flux plot of Fig. 5. Since the total capacity current depends only upon the radio frequency voltage and not upon the circuit configuration, a more uniform distribution of that current is obtained by means of a reduction of current density in the heavy concentration area. This results in a significant improvement in operation because the power loss, and therefore the seal heating varies as the square of the current concentration.

It will thus be seen that I have provided a simple and effective means for improving the operation of high-frequency, high-power apparatus. Purely by way of illustration, tests have shown that the useful radio frequency power output of an amplifier of the type illustrated was increased approximately 50 per cent by the addition of the shields 5| and 52, at frequencies of the order of 100 megacycles.

While the shapes and spacings of the shield plates 5| and 52 are not critical, as previously mentioned, the proportions illustrated have been found satisfactory in actual practice. The plates 5| and 52 preferably extend the entire length of the resonant circuit formed by the transmission line conductors and tube anode structures, and beyond both ends of the circuit, as illustrated in Fig. 2. This prevents the introduction of dis-continuities into the line characteristics. The plates also serve an additional useful function in reducing the transmission line surge impedance. Consequently, the tube inter-electrode capacitances cause less shortening of the physical circuit at a given frequency, and the tubes may therefore be operated at higher frequencies with circuits of practical size. For example, in the particular case of an amplifier operating at about 100 megacycles,

it was found that the physicallength" of the anode circuit was increased from approximately 13.5 inches to 21 inches after the shield plates 5| and 52 were added.

While a particular embodiment of the inven-' tion hasbeen shown and described, it will, of course, be understood that various modifications may be made without departing from the inven-' tion. It is, therefore, intended to cover any such modifications within the true spirit and scope of the invention as defined by the appended claims. What I claim as new and desire to secure by Letters Patent of the .United States is: 1

)1; A: high frequency apparatus comprising, in combination, a resonant circuit including a pair of spaced transmission line conductors and a pair of similarly-spaced electron discharge devices, each of said devices having an electrode structure sealed to the envelope thereof, said electrode structures being positioned adjacent correspond ing ends of said line conductors and electrically connected thereto, means for energizing said apparatus for operation at high frequencies, whereby non-uniformly distributed capacity currents tend to flow over said electrode structures due to non-uniform electrostatic field distribution and thereby to cause localized seal heating, and means for reducing said localized heating comprising a pair of conducting plates spaced from and partially surrounding said pair of structures, said plates being located adjacent areas of normally low electrostatic field density and being connected to a point of neutral potential on said circuit, thereby to increase the field density over such areas and to equalize the distribution of said currents to a substantial extent.

2. A high frequency push pull power amplifier comprising a pair of electron discharge devices each having an externally-cooled anode structure sealed to the envelope thereof, a tuned anode circuit of the open-wire transmission line type including a pair of tubular conductors which are relatively closely spaced in terms of their crosssectional dimensions, each of said anode structures electricaly forming with one of said line conductors a line element of said transmission line, means for energizing said amplifier to produce high frequency anode potentials, whereby non-uniformly distributed capacity currents tend to flow over said line conductors and anode structures due to the concentrated electrostatic field therebetween and thereby to cause localized seal heating, and means for reducing said localized heating comprising a pair of conducting shield plates spaced from and extending axially along the electrically active length of said line elements, said plates being positioned on opposite sides of said transmission line adjacent areas of normally low electrostatic field concentration and maintained at a neutral potential with respect to said anode potentials, thereby to equalize the distribution of said currents to a substantial extent.

3. A high frequency power amplifier comprising a pair of spaced amplifier tubes each having a cylindrical anode sealed to the envelope thereof and an external cylindrical anode structure concentrically surrounding said anode, an anode tank circuit comprising a pair of cylindrical conductors of substantially the same diameter as said anode structures, each conductor supporting one of said structures at one end thereof and forming therewith an element of an open-wire resonant line, said elements being relatively closely spaced in terms of their diameters, means for energizing said amplifier to produce high frequency potentials between said elements, whereby non-uniformly distributed capacity currents tend to flow from said anode structures to said anodes due to the concentrated electrostatic field between said elements and thereby to cause nonuniform seal heating, and means for rendering said heating more uniform comprising a pair of semi-cylindrical conducting shields each concentric with one of said elements and extending ax 10 ially along the electrically active portion thereof, said shields being arranged on opposite sides of said pair of elements and-closely spaced there from in terms of said diameters, and said shields being connected to a point of neutral potential for said high frequency, thereby to equalize the distribution of said currents to a substantial extent.

WILLIAM F. GOETTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,806,281 Davis May 13, 1931 2,247,779 Keister July 1, 1941 2,395,441 Alford Feb. 26, 1946 16 -2,404,188 Phillips July 16, 1946 

